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The Special Senses. P A R T A. Eye and Associated Structures. 70% of all sensory receptors are in the eye Most of the eye is protected by a cushion of fat and the bony orbit Accessory structures include eyebrows, eyelids, conjunctiva, lacrimal apparatus, and extrinsic eye muscles. Eyebrows.

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The Special Senses

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eye and associated structures
Eye and Associated Structures
  • 70% of all sensory receptors are in the eye
  • Most of the eye is protected by a cushion of fat and the bony orbit
  • Accessory structures include eyebrows, eyelids, conjunctiva, lacrimal apparatus, and extrinsic eye muscles
  • Functions include:
    • Shading the eye
    • Preventing perspiration from reaching the eye
palpebrae eyelids
Palpebrae (Eyelids)
  • Protect the eye anteriorly
  • Palpebral fissure – separates eyelids
  • Canthi – medial and lateral angles (commissures)
palpebrae eyelids1
Palpebrae (Eyelids)
  • Lacrimal caruncle – contains glands that secrete a whitish, oily secretion (Sandman’s eye sand)
  • Tarsal plates of connective tissue support the eyelids internally
  • Eyelashes
    • Project from the free margin of each eyelid
    • Initiate reflex blinking
palpebrae eyelids2
Palpebrae (Eyelids)
  • Lubricating glands associated with the eyelids
    • Meibomian glands (modified sebaceous glands)
    • Ciliary glands lie between the hair follicles (sweat and sebaceous glands)
  • Transparent mucous membrane that:
    • Lines the eyelids as the palpebral conjunctiva
    • Covers the whites of the eyes as the ocular or bulbar conjunctiva
    • Lubricates and protects the eye
lacrimal apparatus
Lacrimal Apparatus
  • Consists of the lacrimal gland and associated ducts
  • Lacrimal glands secrete tears
    • Located on the lateral portion of the eye
lacrimal apparatus1
Lacrimal Apparatus
  • Tears
    • Contain mucus, antibodies, and lysozyme
    • Enter the eye via superolateral excretory ducts
    • Exit the eye medially via the lacrimal punctum
    • Drain into lacrimal canaliculus, lacrimal sac and then into the nasolacrimal duct
extrinsic eye muscles
Extrinsic Eye Muscles
  • Six extrinsic eye muscles
    • Enable the eye to follow moving objects
    • Maintain the shape of the eyeball
structure of the eyeball
Structure of the Eyeball
  • A slightly irregular hollow sphere with anterior and posterior poles
  • The wall is composed of three tunics – fibrous, vascular, and sensory
  • The internal cavity is filled with fluids called humors
  • The lens separates the internal cavity into anterior and posterior segments
fibrous tunic
Fibrous Tunic
  • Forms the outermost coat of the eye and is composed of:
    • Opaque sclera (posteriorly)
    • Clear cornea (anteriorly)
  • The sclera protects the eye and anchors extrinsic muscles
  • The cornea lets light enter the eye
vascular tunic or uvea
Vascular Tunic or Uvea
  • Has three regions: choroid, ciliary body, and iris
  • Choroid region
    • A dark brown membrane that forms the posterior portion of the uvea
    • Supplies blood to all eye tunics
vascular tunic or uvea1
Vascular Tunic or Uvea
  • Ciliary body
    • A thickened ring of tissue surrounding the lens
    • Composed of the ciliary muscles (smooth muscle)
      • anchor the suspensory ligament that holds the lens in place
    • Ciliary processes
      • Secrets the aqueous humor
vascular tunic iris
Vascular Tunic: Iris
  • Pupil – central opening of the iris
    • Regulates the amount of light entering the eye during:
      • Close vision and bright light – pupils constrict
      • Distant vision and dim light – pupils dilate
      • Changes in emotional state – pupils dilate when the subject matter is appealing or requires problem-solving skills
sensory tunic retina
Sensory Tunic: Retina
  • A delicate two-layered membrane
  • Pigmented layer – the outer layer that absorbs light and prevents its scattering
  • Neural layer, which contains:
    • Photoreceptors that transduce light energy
    • Bipolar cells and ganglion cells
    • Horizontal and amacrine cells
the retina ganglion cells and the optic disc
The Retina: Ganglion Cells and the Optic Disc
  • Ganglion cell axons:
    • Run along the inner surface of the retina
    • Leave the eye as the optic nerve
  • The optic disc:
    • Is the site where the optic nerve leaves the eye
    • Lacks photoreceptors (the blind spot)
the retina photoreceptors
The Retina: Photoreceptors
  • Rods:
    • Respond to dim light
    • Are used for peripheral vision
  • Cones:
    • Respond to bright light
    • Have high-acuity color vision
    • Macula lutea – mostly cones
    • Fovea centralis– only cones
blood supply to the retina
Blood Supply to the Retina
  • The neural retina receives its blood supply from two sources
    • The outer third receives its blood from the choroid
    • The inner two-thirds is served by the central artery and vein
  • Small vessels radiate out from the optic disc and can be seen with an ophthalmoscope
inner chambers and fluids
Inner Chambers and Fluids
  • The lens separates the internal eye into
  • Anterior segment
    • Anterior chamber
    • Posterior chamber
  • Posterior segment
inner chambers and fluids1
Inner Chambers and Fluids
  • The posterior segment is filled with a clear gel called vitreoushumor that:
    • Transmits light
    • Supports the posterior surface of the lens
    • Holds the neural retina firmly against the pigmented layer
    • Contributes to intraocular pressure
anterior segment
Anterior Segment
  • AnteriorChamber– between the cornea and the iris
  • Posterior Chamber – between the iris and the lens
  • Aqueous humor
    • A plasmalike fluid that fills the anterior segment
    • Drains via the canal of Schlemm
  • Supports, nourishes, and removes wastes
anterior segment1
Anterior Segment

Figure 15.8

  • A biconvex, transparent, flexible, avascular structure that:
    • Allows precise focusing of light onto the retina
    • Is composed of epithelium and lens fibers
  • Lens epithelium – anterior cells that differentiate into lens fibers
  • Lens fibers – cells filled with the transparent protein crystallin
  • With age, the lens becomes more compact and dense and loses its elasticity
  • Electromagnetic radiation – all energy waves from short gamma rays to long radio waves
  • Our eyes respond to a small portion of this spectrum called the visible spectrum
  • Different cones in the retina respond to different wavelengths of the visible spectrum

Figure 15.10

refraction and lenses
Refraction and Lenses
  • When light passes from one transparent medium to another
  • Light passing through a convex lens (as in the eye) is bent so that the rays converge to a focal point
  • When a convex lens forms an image, the image is upside down and reversed right to left
refraction and lenses1
Refraction and Lenses

Figure 15.12a, b

focusing light on the retina
Focusing Light on the Retina
  • Pathway of light entering the eye: cornea, aqueous humor, lens, vitreous humor, and the neural layer of the retina to the photoreceptors
  • Light is refracted:
    • At the cornea
    • Entering the lens
    • Leaving the lens
  • The lens curvature and shape allow for fine focusing of an image
focusing for distant vision
Focusing for Distant Vision
  • Light from a distance needs little adjustment for proper focusing
  • Far point of vision – the distance beyond which the lens does not need to change shape to focus (20 ft.)

Figure 15.13a

focusing for close vision
Focusing for Close Vision
  • Close vision requires:
    • Accommodation – changing the lens shape by ciliary muscles to increase refractory power
    • Constriction – the pupillary reflex constricts the pupils to prevent divergent light rays from entering the eye
    • Convergence – medial rotation of the eyeballs toward the object being viewed
problems of refraction
Problems of Refraction
  • Emmetropic eye – normal eye
  • Myopic eye (nearsighted) – the focal point is in front of the retina
    • Corrected with a concave lens
  • Hyperopic eye (farsighted) – the focal point is behind the retina
    • Corrected with a convex lens
problems of refraction1
Problems of Refraction

Figure 15.14a, b

  • Photoreception – process by which the eye detects light energy
  • Rods and cones contain visual pigments (photopigments)
    • Arranged in a stack of disklike infoldings of the plasma membrane
    • Special epithelial cells - release neurotransmitters that stimulates neurons
  • Sensitive to dim light and best suited for night vision
  • Absorb all wavelengths of visible light
  • Perceived input is in gray tones only
  • Sum of visual input from many rods feeds into a single ganglion cell
  • Results in fuzzy and indistinct images
  • Need bright light for activation (have low sensitivity)
  • Have pigments that furnish a vividly colored view
  • Each cone synapses with a single ganglion cell
  • Vision is detailed and has high resolution
chemistry of visual pigments
Chemistry of Visual Pigments
  • Rhodopsin
  • Retinal is a light-absorbing molecule
    • Synthesized from vitamin A
    • Two isomers: 11-cis and 11-trans
  • Opsins – proteins
    • 4 types that will absorb different wavelengths of light
excitation of rods
Excitation of Rods
  • The visual pigment of rods is rhodopsin (opsin + 11-cis retinal)
  • Light phase
    • Rhodopsin breaks down into all-trans retinal + opsin (bleaching of the pigment)
excitation of rods1
Excitation of Rods
  • Dark phase
    • All-trans retinal converts to 11-cis form
    • 11-cis retinal is also formed from vitamin A
    • 11-cis retinal + opsin regenerate rhodopsin
  • Light energy splits rhodopsin into all-trans retinal, releasing activated opsin
  • The freed opsin activates the G protein transducin
  • Transducin catalyzes activation of phosphodiesterase (PDE)
  • PDE hydrolyzes cGMP to GMP and releases it from sodium channels
  • Without bound cGMP, sodium channels close, the membrane hyperpolarizes, and neurotransmitter cannot be released
signal transmission
Signal Transmission

Figure 15.17b

excitation of cones
Excitation of Cones
  • Visual pigments in cones are similar to rods (retinal + opsins)
  • There are three types of cones: blue, green, and red
  • Intermediate colors are perceived by activation of more than one type of cone
  • Method of excitation is similar to rods
light adaptation
Light Adaptation
  • Going from dark to light:
    • Fast bleaching of rods and cones
      • Glare
    • Rods are turned off
      • Retinal sensitivity is lost
    • Cones are turned on
      • Visual acuity is gained
dark adaptation
Dark Adaptation
  • Going from light to dark:
    • Cones stop functioning and rods pigments have been bleached out by bright light
      • “We see blackness”
    • Rods are turned on
      • Rhodopsin accumulates in the dark and retinal sensitivity is restored
visual pathways
Visual Pathways
  • Axons of retinal ganglion cells form the optic nerve
  • Medial fibers of the optic nerve decussate at the optic chiasm
  • Most fibers of the optic tracts continue to the thalamus
  • Fibers from the thalamus form the optic radiation
  • Optic radiations travel to the visual cortex
visual pathways1
Visual Pathways

Figure 15.19

visual pathways2
Visual Pathways
  • Some nerve fibers send tracts to the midbrain ending in the superior colliculi
  • A small subset of visual fibers contain melanopsin (circadian pigment) which:
    • Mediates pupillary light reflexes
    • Sets daily biorhythms
depth perception
Depth Perception
  • Achieved by both eyes viewing the same image from slightly different angles
  • Three-dimensional vision results from cortical fusion of the slightly different images
  • If only one eye is used, depth perception is lost and the observer must rely on learned clues to determine depth
thalamic processing
Thalamic Processing
  • The thalamus:
    • Relay information on movement
    • Segregate the retinal axons in preparation for depth perception
    • Emphasize visual inputs from regions of high cone density
    • Sharpen the contrast information received by the retina
cortical processing
Cortical Processing
  • Primary visual cortex (striate)
    • Basic dark/bright and contrast information
  • Visual association area (Prestriate)
    • Form, color, and movement
  • Visual information then proceeds anteriorly to the:
    • Temporal lobe – processes identification of objects
    • Parietal cortex and postcentral gyrus – processes spatial location